The Third Section of His Imperial Majesty's Own Chancellery (Russian: Tretiye Otdeleniye, or III отделение собственной Е.И.В канцелярии) was a secret department set up in Imperial Russia, inherited from Tayny Prikaz, Privy Chancellery and Specialty Chancellery. It was designed to combat corruption and champion justice.

The Third Section was organized into the following departments (ekspeditsiya):

• I - supreme police (political crimes and enemies of the regime)
• II - counterfeiting, religious sects, murders, penitentiary, serfdom
• III - aliens
• IV - incidents, staff matters
• V (1842) - censorship of theatre plays (performed by the 1st department since 1828)

Count Alexander Benkendorf was the first Head of the Section from 1826. He was the person who tried to warn Alexander I of the Decembrist plot; thus Nicholas I saw him as perfect head of the secret force. He also served as Chief of Gendarmes, but the office of the Executive Director of the Third Section was not formally merged with Chief of Gendarmes until 1839.

Benkendorf hoped the department would be something of a 'moral physician' to the people; an organisation they trusted and respected. In the beginning, the Gendarmes, often called 'blue archangels' because of their blue and white uniforms, fulfilled this hope - they proved themselves far more efficient at maintaining law and order than the ordinary police.

However, over time its reputation deteriorated as it became progressively easier to imprison than investigate. The Third Section and Gendarmes became associated primarily with the suppression of any liberal ideas as well as strict censorship on printed press and theatre plays. Although only three periodicals were ever banned outright, most were severely edited. It was keen to repress 'dangerous' western liberal ideas, such as constitutional monarchy or even republicanism. Throughout the reign of Nicholas I, thousands of citizens were kept under strict surveillance.

In the end, the powerful image of the Section and the Gendarmes was largely undermined when they failed to suppress the rising revolutionary movement and acts of terrorism against government officials. The large network of informers and agents often supplied nothing more than rumors and slanders. Even the head of the Section Mesentsev fell victim to the assassin in 1878. Following the February 5, 1880 assassination attempt on Alexander II, both the Third Section and the Corps of Gendarmes were subordinated to the Supreme Executive Comission under Count Loris-Melikov, who then advised the dismissal of the Third Section. The Corps of Gendarmes was then transferred to the Ministry of Internal Affairs' Department of Police.

Heads of the Third Section

• Count Alexander Khristoforovich Benkendorf (1826-1844)
• Count Alexey Fyodorovich Orlov (1844-1856)
• Prince Vasily Andreyevich Dolgorukov (1856-1866)
• Count Pyotr Andreyevich Shuvalov (1866-1874)
• Alexander Lvovich Potapov (1874-1876)
• Nikolay Vladimirovich Mezentsev (1876-1878)
• Alexander Romanovich Drenteln (1878-1880).